Finally, the right combo of sports to make you race-fit, faster than you ever imagined.

Text by Devon O'Neil

From the time he was a teenager, Justin Burger spent most of his free time, 20 hours a week or more, training for endurance races—ultramarathons, 24-hour mountain bike competitions, 12-hour kayak races. His hard work paid off. The 23-year-old Houston-based personal trainer routinely finished near the top of the podium at premier adventure races across Texas. "After I started college, though, I just had less time to exercise," he says, "so I expected the worst." Conventional fitness wisdom told him his results would plummet when his workout time was halved. Instead, he got faster.

Next: Less is More

Less is More

The benefits of cross-training have been well-known since Nike aired those "Bo Knows" commercials a generation ago: A combination of different activities conditions the entire body—not just specific muscle groups utilized for one sport—and prevents injury by allowing some muscles to recover while others are pushed. But only now are we starting to grasp the best way to put cross-training to use. A multisport routine can get you in the best shape of your life—in six weeks or less. All it takes is a month and a half of long workouts to build a base that’s strong enough to sustain you in the long run, with a much smaller time commitment.

The idea is rapidly gaining traction among athletes, but the principles of Burger’s do-less success apply to mortals too. With a solid fitness foundation, you can prep for a big event—a marathon, a century, even an Ironman—in far less time than most people realize. In Burger’s case, he was already fit, so now he no longer needs to invest hours each day to maintain his edge. In fact, he says, he stays competitive by "running just 45 minutes on weekdays and then doing a different activity for longer bouts on weekends."

The best plan of attack? Alternate short, strenuous workouts featuring different activities (see "How to Get More Out of Doing Less," below). "The key is intensity," says Bill Pierce, a founder of the Furman Institute of Running & Scientific Training and co-author of the training guide Run Less, Run Faster. "If you run hard one day, for instance, you can’t run hard again the next because those running muscles need to recover. But if you, say, cycle the next day instead, you can go hard again. Your cycling muscles will get an intense workout while your running muscles recover. So you’re able to prevent injury—and achieve total fitness in a lot less time."

Next: The No-Pain Gain

The No-Pain Gain

Hard-core athletes often suffer from the same chronic pains—swimmer’s shoulder, runner’s knee, tennis elbow—because every sport employs muscles in unique ways. Besides saving time, the new model of short, hard, and varied workouts also limits the stress that occurs on specific muscle groups. Peter Janes, an orthopedic surgeon in Summit County, Colorado, has worked with multisport athletes for 25 years and says cross-training of any length or intensity significantly reduces the risk of overuse injuries, particularly tendinitis. "When you’re using only a few muscle groups over and over again, the tendons will break down and become inflamed or even rupture," he says. "Cross-training with a number of sports forces you to use muscle-tendon units throughout your body."

Kristen Dieffenbach, Ph.D., a West Virginia University professor of athletic coaching and education, favors cross-training because of its above-the-neck advantages: "Switching between activities is more stimulating," she says. "It’s human nature. You’re less likely to get bored." Dieffenbach competes nationally in multisport races such as Primal Quest, a 500-mile beast where she placed in the top ten last summer. Cross-training, she says, keeps her mental capacities fresh. "For me, biking requires quite a bit of concentration, so it’s a mental workout as much as a physical one," she says. "But when I’m running or paddling, I can operate on autopilot a bit more and save up my mental energy for later."

Next: Freedom of Choice

Freedom of Choice

The list of á la carte cross-training options is almost endless. But certain types of athletes tend to settle on certain groups of activities. A runner might mix Pilates and cycling with sprint workouts. A masters-level swimmer is likely to add a day or two of weight training to pool work. Nike-sponsored world champion adventure racer Monique Merrill, 39, prefers to schedule a few mini-workouts throughout the day. She works 45 hours a week at the natural foods store she owns in Breckenridge, Colorado, so to break up her workday, she’ll go for a trail run in the morning, a quick mountain bike ride at lunch, and then another run after work. "It’s what soothes me," she says. "It’s a great midday stress reliever, and I’m able to fit everything in." She saves her longer adventures for the weekend—often trekking in the mountains near her house—but the short stuff helps her just as much. "The attitude has definitely switched in the last year," says Chance Reading, a U.S. Adventure Racing Association director. "People are getting faster with shorter, harder workouts."